I would imagine it hurts like it does for humans. Their eyes are adapted for darkness and they instinctively hide from bright light. Not sure if it's ever been shown scientifically to cause harm but I just assume that it does. They can't close their eyes I never shine any kind of light at my BP's eyes that I wouldn't be comfortable staring at directly myself. Since he's an albino and he lacks melanin to protect his eyes I am especially careful with exposing him to bright light and I try to avoid it whenever possible.